Nuclear Magnetic Resonance Spectroscopy. Conformational Properties of Cyclobutanes. Variation of Vicinal Hydrogen-Fluorine Coupling Constants with Temperature' Joseph B. Lambert2 and John D. Roberts
Contribution No. 3230 f r o m the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California. Received M a y 12, 1965 The observation of substantial temperature variations of the vicinal hydrogen-fluorine coupling constants in a series of cyclobutanes is consistent with the existence of an equilibrium between conformations. The temperature dependencies are discussed in terms of the models developed previously. The magnitude of these couplings as a function of the dihedral angle appears to follow a relationship similar to that described by Karplus for proton-proton couplings. Introduction The n.m.r. method discussed in the previous paper3 provides a clear demonstration of the presence or absence of nonplanarity in difluorocyclobutanes, and the dipole moment method furnishes a reasonably accurate indication of the degree of puckering. The model of interconverting conformers was adopted by analogy with the properties of cyclohexane systems, but the size of the barrier is such that the model is more closely related to that of rapidly interconverting ethane rotamers. The model is valid provided that the conformers form ideal solutions with each other and have the same entropy and heat capacity over the observed temperature range. When this is the case, the observed molecular properties are weighted averages of the properties of the individual conformers. Changes in the populations with temperature will therefore bring about changes in observables such as the chemical-shift differences. Although the conformational properties of cyclobutanes were derived entirely from temperature-dependent fluorine-fluorine chemical-shift differences, there should be discernible changes in the vicinal hydrogen-fluorine coupling constants as well. Such behavior has been noted extensively in studies of the equilibria among rotamers of substituted ethanes (eq. 1) 4-7 for hydrogen-hydrogen, hydrogen-fluorine, F
F
F
F
c1
F
(1) Supported in part by the Office of Naval Research and the National Science Foundation. (2) National Science Foundation Predoctoral Fellow, 1962-1965. (3) J. B. Lambert and J. D. Roberts, J . Am. Chem. Soc., 87, 3884 (1965). (4) R. W. Fessenden and J. S. Waugh, J . Chem. Phys., 37, 1466 (1962). (5) R. J. Abraham and H. J. Bernstein, Can.J . Chem., 39, 39 (1961). (6) H. S. Gutowsky, G. G. Belford, and P. E. McMahon, J . Chem. Phys., 36, 3353 (1962). ( 7 ) H . S . Gutowsky, Pure A&. Chem., 7 , 93 (1963).
and fluorine-fluorine couplings, as well as in less general rotational equilibria (eq. 2-4).8-10 Small temperature variations have, however, been observed for
F
\
F
/F
?="\
F'O F
.
. \/"="\IF /FF 0
fluorine-fluorine couplings in systems, such as I and 11, which are incapable of rotational isomerism. The CF3CFCIz I
CF3CFzCOzH
I1
torsional oscillations which give rise to these changes appear to be less important for hydrogen-hydrogen and hydrogen-fluorine couplings. Thus, in system 2, couplings between nuclei which are attached to the conformationally rigid double bond exhibit temperature variations which are negligibly small in comparison to the changes in the couplings between the vinyl protons and nuclei in the perfluoroisopropyl substituent. If it is true that the large variations in the fluorinefluorine chemical-shift differences are due to conformational changes rather than torsional oscillations, then substantial variations should also be observable in the vicinal hydrogen-fluorine coupling constants, to which torsional oscillations appear to contribute negligibly, Observation of such variations would also comprise one of the first examples of temperature-dependent coupling constants other than in rotational equilibria. 1 2 (8) N. Boden, J. W. Enelin, J. Feeney, and L. H. Sutcliff, Proc. Roy. Soc. (London), A282, 559 (1964). (9) W. S. Brey and K. C. Ramey, J. Chem. Phys., 39, 844 (1963). (10) J. JonAH and H. S. Gutowsky, ibid., 42, 140 (1965). (11) J. C . Schug, P. E. McMahon, and H. S. Gutowsky, ibid., 33, 843 (1960). (12) Dudek and Dudek have, however, observed coupling constants
with a temperature dependence derived from a totally different process; cf. G. 0. Dudek and E. P. Dudek, J . Am. Chem. Soc., 86, 4283 (1964).
Lambert, Roberts / Temperature Variation of Vicinal H-F Couphng Constants
3891
looo
1
-50‘ 30“
h
Figure 1. Calculated (-50, 30, 100’) and experimental (100”) 60Mc.p.s. proton spectrum of l,l-difluoro-2,2-dichloro-3-deuterio-3phenylcyclobutane (111), with simultaneous irradiation at the deuterium frequency.
Results and Discussion The complete analysis of the hydrogen-fluorine spectra of the ring nuclei of four of the cyclobutanes considered in the previous study3 (111-VI) was carried out at -50, 30, and 100”. In no case was the spectrum
!,111 -50’
r!!
!
Figure 2. Calculated 56.4-Mc.p.s. fluorine spectrum of 1,ldifluoro-2,2-dichloro-3-deuterio-3-phenylcyclobutane (111).
c1, ,c1
““““>O
